Since a speed changing mechanism which includes planet gears has a simple construction and the input and output shafts of the mechanism can be disposed coaxially, the mechanism has been widely used as a speed reducer for an electric rotary tool such as an electric screwdriver, an electric screw tightener or a vibratory drill. Such speed changing mechanisms are disclosed in Japanese Patent Unexamined Publication Nos. Sho. 62-173180 and 62-297006. Due to the gear combination of each of these conventional speed changers, the ratio of the rapid rotation speed of the mechanism to the slow rotation speed thereof is 2.5 or greater, and cannot be set to a value as small as 1.5.
There has been developed an electric press-attaching tool including an automatic speed changing mechanism, which is powered by a battery, and a hydraulic pump, which is driven by the mechanism. The tool need not have a separately disposed pump, a separately laid pressure-proof hose or a manipulation lever for increasing the working oil pressure. The tool's operation is unrestricted by the movement of the user thereof, and is easily handled.
If a force for pressing and deforming a press-attachable terminal or sleeve or the like by the tool is to be weak, such as when connecting thin electric wires or a small number thereof together by the terminal or sleeve or the like, the tool's rotational frequency is increased, thereby decreasing the output torque thereof and shortening the tool's working time. In contrast, if a strong force is required such as when connecting thick electric wires or a large number of electric wires together by the terminal or sleeve or the like, the tool's rotational frequency is automatically decreased, thereby increasing the output torque thereof. Thus, the force magnitude can be adjusted appropriately for the work to be performed by the tool.
In such an automatic speed changer which can increase its rotational frequency to decrease the output torque thereof and shorten working time and which can decrease its rotational frequency to increase the output torque, to efficiently apply an appropriate working force, a ratio of the mechanism's rapid rotation speed to the slow rotation speed thereof must be set to approximately 1.5. As discussed above, the conventional speed changing mechanisms do not meet this requirement, and thus a problem arises in that an appropriate working force is not applied to a workpiece.